This invention relates to a film forming composition comprising a blend of at least two aqueous nitrile resin latexes which differ in film forming characteristics and in dry film properties with the resultant blend exhibiting surprisingly superior surface coating properties.
Synthetic copolymers and interpolymers of monomer mixtures comprising acrylonitrile and acrylate esters are well known and some of them have well-established commercial utility. Particularly useful are the high acrylonitrile copolymers containing 80% by weight or more of acrylonitrile and the low acrylonitrile copolymers containing 20% by weight or less of acrylonitrile. The high acrylonitrile copolymers and interpolymers are most generally used in the manufacture of synthetic fibers and the low acrylonitrile copolymers and interpolymers are useful as elastomers. More details concerning the state of the art of the aforementioned acrylonitrile interpolymers may be found, for instance, in U.S. Pat. Nos. 2,412,034, 2,558,396, 2,688,608, and 2,958,670, as well as Industrial and Engineering Chemistry, vol. 47, pages 1000 et seq. (1955), vol. 46, pages 1992 et seq. (1954), vol. 43, pages 2500 et seq. (1951), Journal of the Americal Chemical Society, vol. 65, pages 2067 et seq. (1943), and Rubber Age, February 1953, pages 636-637.
It is well known that it is difficult to prepare films at normal temperatures from a latex resulting from the polymerization or copolymerization of high proportions of acrylonitrile as disclosed in U.S. Pat. No. 3,047,522. Improved nitrile latexes have been described which are prepared by the polymerization of a major proportion of an olefinically unsaturated nitrile with a monomer component copolymerizable therewith optionally in the presence of preformed diene rubber. For example, see U.S. Pat. Nos. 3,426,102; 4,244,852; and 4,329,401. The high concentration of the nitrile component results in the film being an excellent oxygen barrier. Further advantages of a high nitrile film are hardness, toughness, abrasion resistance, solvent resistance and adhesion to metal substrates. However, these high nitrile latexes characteristically form films at rather high temperatures, and they require long times to produce films which are pin hole free.
When latex dispersions are intended to be used as coating materials, it is desirable that the glass transition temperature (Tg) of the resin phase be such that particle coalescence and film formation occur at temperatures conventionally used for the application and baking of surface coatings in commercial equipment. For energy conservation, the application and finishing temperatures should be as low as possible. The minimum temperature at which a latex coalesces and forms a continuous or integral film is referred to as the minimum film forming temperature (MFT).
Applicants have discovered that the film forming temperature and the time required to produce a film can be significantly lowered by blending a high nitrile latex with a second latex material that has a lower minimum film forming temperature (MFT) without the loss of the desirable and useful film properties.